A história do romance entre o carismático poeta Percy Shelley e Mary Wollstonecraft, uma jovem de 17 anos que viria a se tornar a aclamada escritora Mary Shelley.

Living on an estate on the shores of Lake Geneva, Lord Byron is visited by Percy and Mary Shelley. Together with Byron's lover Claire Clairmont, and aided by hallucinogenic substances, they devise an evening of ghoulish tales. However, when confronted by horrors, ostensibly of their own creation, it becomes difficult to tell apparition from reality.

Lady Caroline Lamb, dissatisfied in her marriage, has an affair with the dashing Romantic poet Lord Byron.

In the summer of 1816, Percy Shelley, his mistress Mary, and her stepsister Claire visit Lord Byron at Lake Geneva. Byron challenges each to write a horror story, and Mary begins her novel, Frankenstein. She imagines the monster becoming real, and for the next six years, as tragedy befalls those around her, she believes the personification of her imagination is the cause. Against this backdrop, Claire has Byron's baby then is estranged from him and barred from her daughter. Byron and Percy continue their friendship, the one self-centered and decadent, the other wildly idealistic. The Shelleys take up residence near Pisa.

Authors Lord Byron, Mary Shelley (née Godwin) and Percy Shelley get together for some philosophical discussions, but the situation soon deteriorates into mind games, drugs, and sex. It is a fictionalization of the summer that Lord Byron and the Shelleys, together with Lord Byron's ex-lover Claire Clairmont and his Doctor John Polidori, spent in the isolated Villa Diodati by Lake Geneva. It is there they devise a contest to adduce the best horror story to kill the dullness of summer. It is also there that one of the world's most famous books was given life—Mary Shelley's Frankenstein.